You can go smaller then the Jetcat chart. I run 40mm on "P20", even up to 35N. Jetcat says 45mm. I run 46mm on a 55N engine, Jetcat says 65mm for 60N....

I happen to just have tested a bifurcated thrust tube where I have a Kolibri T25 making 2.8kg, with dual 28mm outlets I got 2.25kg out, with dual 30mm I got 1.9kg. Bigger got significantly less(!) jetcat says dual 40mm?!....

With that said I can not help on a 100N engine, but smaller is usually much better and runs cooler. Even though you would guess the opposite.